My work gave me an IP Based Phone (Mitel 5212) and it uses SIP. I have an adapter that uses Power Over Ethernet (POE) and it's attached to my router.
I want to take this phone and reprogram it to use VOIP in a different way. However, I have no experience with these phones.

When I was looking on Google and I found Asterisk (which is an open source Software Development Kit) for VOIP, but I don't know how all of this works.
I can install the Asterisk on a Virtual Machine in Linux using VirtualBox, but I wanted to see if anyone here has played with anything like this.